In many integrated circuit technologies, a short between a signal line and one of ground or a power supply can cause the signal line to be “stuck at” a fixed voltage level. Other manufacturing flaws can cause a switch to be “stuck open”, “stuck closed”, or generate an erroneous output for a given set of inputs. For analog circuitry, faults may present themselves as an erroneous impedance, driver output strength, and receiver offset. Other errors are possible. Built in self test (“BIST”) is a circuit design technique in which physical elements of a circuit are devoted to testing the circuit itself to identify, for example, stuck at faults. Input/output BIST (“IBIST”) is a design technique for testing input/output (“I/O”) circuitry.
FIG. 1 is a block diagram illustrating a central processing unit (“CPU”) 100 having a processor core for executing software instructions coupled via a datapath to an I/O unit for communicating with devices external to the CPU. CPU 100 includes IBIST logic 105 that may be internal to the I/O unit or integrated along side the I/O unit. IBIST logic 105 is physical test circuitry cast into the silicon of CPU 100 along side operation circuitry for the express purpose of testing the operation circuitry of the I/O unit. IBIST logic 105 may include logic to directly stimulate the I/O unit with test vectors strategically designed to test certain portions of the I/O unit to determine whether these portions contain a manufacturing flaw.